Polytetrafluoroethylene or PTFE is known to have a unique combination of properties including excellent chemical resistance, thermal stability at high temperature, low surface energy and excellent electrical (dielectric) properties. PTFE is also known to have two deficiencies which limit its use: high cold flow or creep and poor resistance to ionizing radiation.
Stretching certain forms of PTFE to make microporous expanded PTFE (ePTFE) can improve strength, decrease cold flow or creep, and improve the dielectric properties without changing surface or chemical properties. The chemical resistance or inertness, as well as the low surface energy, of PTFE and ePTFE are beneficial properties for some applications. But for other applications, it would be beneficial to selectively modify these properties without degrading the polymer.
There has been significant research to modify the surface or chemical properties of PTFE and microporous ePTFE in order to improve adhesion and compatibility with other materials. For example, efforts have included attempts to decrease creep by radiation crosslinking, increase or decrease the surface free energy (e.g. increase or decrease hydrophilicity), and provide sites for chemical reactions to improve the utility of PTFE and/or ePTFE in specific applications by chemical and plasma treatments.
Recently, plasma treatment of microporous ePTFE in the presence of maleic anhydride is reported to have produced acid functionality on the surface of the microporous ePTFE. Though the exact mechanism of these surface reactions is not reported, it likely results from the formation of free radicals by bond scission. Where carbon-carbon bond strength is known to be about forty percent lower than carbon-fluorine bonds, a majority of the radicals would result from scission of the carbon-carbon bonds, or main polymer chain scission, thereby decreasing the molecular weight of the polymer, and restricting the anhydride or acid functionality to the ends of the degraded polymer chains. Plasma graft polymerization is restricted near the surface of the sample. (Plasma Surface Modification and Plasma Polymerization; N. Inagoki, Technomic Publishing, 1996, p. 44).
Techniques for dispersion polymerization of both tetrafluoroethylene (TFE) monomer and TFE copolymers have been described. There are references which define and distinguish TFE copolymers based on the concentration of the comonomer. TFE polymers containing less than 1 weight percent comonomer have been referred to as modified homopolymers, or modified PTFE, whereas TFE polymers containing more than 1 weight percent comonomer have been referred to as TFE copolymers. (Fluoroplastics—Vol 1: Non-Melt Processible Fluoroplastics; Williams Andrew, Inc., Norwich, N.Y., at p. 19 (2000).) However, for use herein, TFE polymers containing one or more comonomers at any concentration will be defined as TFE copolymers.
In some processes, dispersion polymerization of TFE produces a resin that has come to be known as “fine powder.” (e.g., U.S. Pat. No. 4,016,345 (Holmes, 1977)). Generally, in such processes, sufficient dispersing agent is introduced into a water carrier such that, upon addition of TFE monomer in the presence of a suitable polymerization initiator and, upon agitation and under autogenous TFE pressure of 10-40 kg/cm2, polymerization proceeds until the desired level of colloidally dispersed polymer particles is reached and the reaction is then stopped. The dispersed polymer particles may subsequently be coagulated by known techniques to obtain the fine powder form of the polymer. The fine powders are dried at temperatures from about 100 to 200° C.
Fine powder resins are known to be useful in paste extrusion processes and in stretching (expansion) processes in which the paste-extruded extrudate, after removal of extrusion aid lubricant, is stretched to produce porous, strong products of various cross-sectional shapes such as rods, filaments, sheets, tubes, etc. Such a stretching process is disclosed in commonly owned U.S. Pat. No. 3,953,566 (“'566” to Gore). The expansion process as it applies to fluorocarbon polymers is described in the aforesaid '566 patent. As used herein, articles that can be the expanded by the process of the '566 patent are said to be “expanded” and the resins used in the expansion process to create such articles are said to be expandable TFE polymers or expandable TFE copolymers.
Dispersion processes to make TFE copolymers are taught in for example, in U.S. Pat. No. 4,792,594 (Gangal et al.), U.S. Pat. No. 6,541,589 (Baillie), U.S. Pat. App. 2007/0010642 (Sabol and Baillie) and U.S. patent application Ser. No. 11/906,877 (Ford; filed Oct. 4, 2007). Dispersion processes to make copolymers are also described. It is taught that fine powders made by these dispersions may be paste extruded and processed by the processes disclosed in U.S. Pat. No. 3,953,566 to make microporous expanded products. TFE fine powder polymer processed by paste extrusion or expansion has high crystallinity especially for the portion of polymer formed in the later stage of the polymerization. This material is sometimes described as the shell or the sheath of the dispersion particle.
TFE copolymers processable by melt extrusion and injection molding include TFE-HFP (hexafluoropropylene) copolymers known as FEP, TFE perfluoroalkyl vinyl ether copolymers known as PFA and MFA, and TFE ethylene copolymers known as E-TFE. These polymers are not fine powders and cannot be paste extruded or expanded into microporous products because of low crystallinity.
TFE copolymers made from fluorovinyl ether comonomers having sulfonyl fluoride groups, ester groups and cyano groups have been described having the formulae:CF2═CF—ORfSO2F  I.CF2═CF—ORfCOOCH3  II.CF2═CF—ORf—CN  III.where Rf is fluoroalkyl or fluoroalkyl ether. (Fluoroplastics—Vol. 2: Melt Processible Fluoropolymers; Williams Andrew Inc.; Perfluorinated Ionomer Membranes, American Chemical Society Symposium, Series 180, 1982; U.S. Pat. No. 3,692,569 (Grot); Moore, Albert L. Fluoroelastomers Handbook, William Andrew Publishing, 2006)Monomers of structures I and II are copolymerized with TFE to form polymers subsequently hydrolyzed to form the sulfonic acid and carboxylic acid. However, these polymers contain sufficient concentration of comonomer that there is little if any crystallinity in the polymers. Monomers of structure III have been polymerized with TFE and perfluoroalkylvinyl ethers to make perfluoro elastomers where monomer with structure III is the cross link site for the elastomers. The materials have little or no crystallinity and are therefore not expandable to create microporous materials.
U.S. Pat. App. 2006/0270780 (Xu et al.) teaches a PTFE modified with a cyanovinyl ether cross linking monomer in a microemulsion process. In this patent application, the modified PTFE is not a fine powder and cannot be paste extruded and expanded according to the '566 process.
U.S. Pat. No. 7,019,083 (Grootaert) teaches low molecular weight melt processable TFE perfluoropropylvinyl ether (PPVE) copolymer containing a cyanovinyl ether that is not formed as a fine powder and which would lack sufficient crystallinity to be paste extruded and be processed into microporous products. U.S. Pat. No. 4,326,046 (Miyaka) teaches making modified PTFE by including 0.001 to 10 mol % of a comonomer component having an acid type functional (or precursor to an acid) group. The acid includes carboxylic, sulphonic or phosphoric acids. U.S. Pat. No. 4,326,046, teaches that the particle of the modified polytetrafluoroethylene comprises a core made of homopolymer of tetrafluoroethylene and the modifier component is included in the sheath layer. U.S. Pat. No. 4,326,046 does not teach paste extruding or expanding the modified polymer. Materials having the high modifier component polymerized in the later stages of polymerization would not have sufficient crystallinity to be processed into microporous products by the '566 process.
U.S. Pat. No. 7,342,066 to Dadalas et al. teaches use of a PTFE dispersion in a coating process. The PTFE contains up to 1 weight percent of an ionic comonomer (e.g., a monomer having acid groups as an ionic group) where at least a portion and preferably all of the comonomer is added in the later stage of the polymerization. U.S. Pat. No. 7,342,066 does not teach forming a paste extrudable or expandable fine powder. Materials made with the high comonomer concentration at the later stages of polymerization would have low crystallinity and would not be paste extrudable or expanded by the processes of the '566 patent.
There is a need for TFE copolymer materials containing functional groups which impart specific chemical properties to a polymer, wherein the copolymer can be expanded to provide a microstructure characterized by nodes interconnected by fibrils. There is a further need for expanded TFE copolymer materials containing functional groups that can undergo subsequent controlled reactions to impart other specific properties to the expanded material while maintaining properties of expanded TFE copolymer material.